Alum Rock in Winter
In winter (December, January and February), Alum Rock sees average daytime highs around 57°F and overnight lows near 41°F, with 12.8 in of rain over about 27 wet days across the season. It is the coldest season of the year and the wettest season of the year.
Across winter, daytime highs hold fairly steady around 57°F. The sky is clear or mostly clear about 35% of the time across winter, and the air most often feels dry.
Winter is Alum Rock's weakest season of the year to visit on our travel score. For the whole year in context, see Alum Rock's year-round climate.
- Avg daytime high
- 57°F
- coldest season of the year
- Avg overnight low
- 41°F
- Rainfall
- 12.8 in
- 27 wet days
- Avg wind
- 4 mph
- Travel score
- 4.2
- weakest season of the year
- Months
- Dec · Jan · Feb
Temperature in Alum Rock in Winter
Across winter, daytime highs hold fairly steady around 57°F.
A typical winter day in Alum Rock reaches about 57°F and drops to 41°F overnight. The warmest of the three months is February, the coolest December.
Alum Rock weather by month
How the three winter months (December, January and February) compare with the rest of the year — the season's months are highlighted.
| Month | High / Low (°F) | Rainfall (inches) | Wet days | Daylight (hours) | Travel score |
|---|---|---|---|---|---|
| Jan | 56° / 40° | 4.4 | 9.1 | 9.7 | 4.1 |
| Feb | 58° / 42° | 4.4 | 9.2 | 10.6 | 4.4 |
| Mar | 61° / 44° | 3.3 | 9.2 | 11.7 | 5.4 |
| Apr | 65° / 45° | 1.6 | 5.7 | 12.9 | 6.7 |
| May | 71° / 49° | 0.8 | 3.4 | 14 | 8.1 |
| Jun | 77° / 53° | 0.2 | 0.8 | 14.5 | 9.1 |
| Jul | 82° / 57° | 0.1 | 0.2 | 14.3 | 8.8 |
| Aug | 82° / 57° | 0.0 | 0.3 | 13.4 | 8.8 |
| Sep | 80° / 56° | 0.1 | 0.6 | 12.3 | 8.9 |
| Oct | 73° / 51° | 1.0 | 2.8 | 11 | 8.6 |
| Nov | 63° / 44° | 1.9 | 5.7 | 10 | 6.2 |
| Dec | 56° / 40° | 3.9 | 8.8 | 9.5 | 4.0 |
Cloud cover in Winter
The sky is clear or mostly clear about 35% of the time across winter.
Chance of precipitation in Winter
Alum Rock gets around 12.8 in of rain over 27 wet days across winter. The line is the day-by-day chance of measurable precipitation.
Humidity in Winter
Most of winter feels dry — the stacked bands show each day's mix of comfort levels, read off the dew point.
Wind in Winter
Winds average 4 mph across winter.
Sunrise & sunset in Winter
Daylight runs from about 9 h 36 min in early December to 11 h 06 min by late February. The lit band spans sunrise to sunset each day.
Solar energy in Winter
Average shortwave solar energy reaching the ground each day across winter, about 2.8 kWh/m².
Hazard calendar for Alum Rock
In winter, Alum Rock overlaps its rainy season season. The full year is below, with each hazard's peak months called out.
Alum Rock's rainy season runs December–March, when most of the year's rain falls.
Is winter a good time to visit Alum Rock?
Our two travel scores rate winter for general sightseeing and for beach or pool weather — 0 (poor) to 10 (ideal), averaged across the three months.
winter is a reasonable time to visit Alum Rock for sightseeing.
What to pack for Alum Rock in Winter
From winter's temperature range, rain chance, sun and wind — what's worth putting in the bag.
In winter, Alum Rock sees daytime highs around 57°F and nights near 41°F. Worth packing: a warm layer and rain gear. About 27 days see rain.
Sea temperature near Alum Rock in Winter
The nearest coast averages about 54°F across winter — the other half of the beach question. The full-year curve and swim-comfort zones are below.
The sea at the nearest coast (about 30 mi away) is warmest in August at about 59°F and coldest in April near 54°F.
The water stays below 68°F all year — cool enough that most swimmers want a wetsuit.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sea temperature (°F) | 54 | 54 | 54 | 54 | 54 | 55 | 57 | 59 | 59 | 58 | 56 | 55 |
| Comfort | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing | Bracing |
| Sea temperature (°F) | Comfort | |
|---|---|---|
| Jan | 54 | Bracing |
| Feb | 54 | Bracing |
| Mar | 54 | Bracing |
| Apr | 54 | Bracing |
| May | 54 | Bracing |
| Jun | 55 | Bracing |
| Jul | 57 | Bracing |
| Aug | 59 | Bracing |
| Sep | 59 | Bracing |
| Oct | 58 | Bracing |
| Nov | 56 | Bracing |
| Dec | 55 | Bracing |
UV index in Alum Rock in Winter
Clear-sky midday UV across winter averages around 3. The full-year curve, shaded by WHO exposure level, is below.
Under clear skies, the midday UV index in Alum Rock peaks around June 23 at about 10 (extreme) — sunscreen, a hat and midday shade are essential. It bottoms out near December 15 at about 2 (moderate).
The index reaches 3 or more — the level where the WHO recommends sun protection — in 9 months of the year.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| UV index (midday) | 3 | 4 | 5 | 7 | 8 | 10 | 10 | 10 | 6 | 5 | 2 | 2 |
| Level | Moderate | Moderate | Moderate | High | Very high | Extreme | Extreme | Very high | High | High | Moderate | Moderate |
| UV index (midday) | Level | |
|---|---|---|
| Jan | 3 | Moderate |
| Feb | 4 | Moderate |
| Mar | 5 | Moderate |
| Apr | 7 | High |
| May | 8 | Very high |
| Jun | 10 | Extreme |
| Jul | 10 | Extreme |
| Aug | 10 | Very high |
| Sep | 6 | High |
| Oct | 5 | High |
| Nov | 2 | Moderate |
| Dec | 2 | Moderate |
Air quality in Alum Rock in Winter
Average fine-particle pollution (PM2.5) through the year — so you can see where winter falls, not just the annual average.
Air quality in Alum Rock is worst in August — around 34 µg/m³ PM2.5 (moderate), about 4.9× the cleanest month (June, 7 µg/m³).
Modeled monthly averages (CAMS reanalysis, 2015–2024).
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PM2.5 (µg/m³) | 13 | 13 | 13 | 10 | 8 | 7 | 9 | 34 | 16 | 14 | 15 | 12 |
| Level | Moderate | Moderate | Moderate | Good | Good | Good | Good | Moderate | Moderate | Moderate | Moderate | Good |
| PM2.5 (µg/m³) | Level | |
|---|---|---|
| Jan | 13 | Moderate |
| Feb | 13 | Moderate |
| Mar | 13 | Moderate |
| Apr | 10 | Good |
| May | 8 | Good |
| Jun | 7 | Good |
| Jul | 9 | Good |
| Aug | 34 | Moderate |
| Sep | 16 | Moderate |
| Oct | 14 | Moderate |
| Nov | 15 | Moderate |
| Dec | 12 | Good |
Where is Alum Rock?
Alum Rock sits at 37.4°N, 121.8°W, 151 ft above sea level, in United States. The nearest listed city is San Jose, 4.3 mi away.
Topography around Alum Rock
How much the land rises and falls, and what covers it, within 2, 10 and 50 miles of Alum Rock.
Within 2 miles, the terrain around Alum Rock has significant variations in elevation — a maximum elevation change of 1,129 ft around an average of 240 ft above sea level; within 10 miles, large variations in elevation — a maximum elevation change of 3,921 ft; within 50 miles, very significant variations in elevation — a maximum elevation change of 4,373 ft.
The land around Alum Rock is covered by artificial surfaces (73%) within 2 miles, artificial surfaces (42%), grassland (33%) and trees (22%) within 10 miles, and grassland (29%), trees (25%) and water (25%) within 50 miles.
United States travel facts
- Power plugs
- Type A / B
- 120 V · 60 Hz
- Prices
- ≈ 100% of US
- price level, 2025
- GDP per capita
- $90,027
- 2025
- Happiness
- 6.8 / 10
- World Happiness, 2025
- Safe drinking water
- 98%
- of population, 2024
- Homicide rate
- 5.8
- per 100k, 2023
- Road deaths
- 12.7
- per 100k/yr, 2019
- Income group
- High
- World Bank
- Religion
Sources: World Bank, US State Department, World Happiness Report, WHO/UNICEF, EF EPI, CIA World Factbook.
Data sources
Temperature, precipitation, cloud, humidity, wind and solar figures are a 1991–2020 climatology for Alum Rock (37.3661°, -121.8272°, elevation 151 ft), computed from hourly ERA5 reanalysis — contains modified Copernicus Climate Change Service information. Sun figures are astronomical calculations, not observations. City data © GeoNames, CC BY 4.0. Map outlines from Natural Earth (public domain); interactive map © Google.